.arcconfig

extensions: import the exthelper class from evolve 980565468003 (API)

This should help make extensions that wrap a lot of stuff more comprehendible.
It was copied unmodified, except:

  - fix up the imports
  - rename final_xxxsetup() -> finalxxxsetup() to appease checkcode
  - avoid a [] default arg to wrapcommand()

.. api::

   Add `exthelper` class to simplify extension writing by allowing functions,
   commands, and configitems to be registered via annotations.  The previous
   APIs are still available for use.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}